Air pollution is one of the main environmental problems of the XXIst Century. After the publication of the Intergovernmental Panel on Climate Changes (IPCC) Report in 2007 [1], global warming is considered to be the responsible for climate changes in the planet [2–10] and for bringing significant consequences to human society.

The same Report highlights sulfur hexafluoride (SF₆) as an important greenhouse gas, as it has a radiative forcing of 0.52 W/m², an atmospheric lifetime of 3,200 years and a Global Warming Potential (GWP) of 23,900. Sulfur hexafluoride is almost non-reactive and therefore it is widely employed for insulation and current interruption purposes in electric power transmission and distribution equipment. It is also used in the magnesium industry to protect molten magnesium from oxidation and potential violent burning, in semiconductor manufacturing to create circuitry patterns on silicon wafers and as a tracer gas for leak detection [11].

In trace-gas analysis of chemical species in the atmosphere, besides sensitivity and selectivity the response time is of increasing interest for the real-time detection of temporal concentration changes. In order to achieve the necessary sensitivity and selectivity, the use of high-resolution laser techniques in IR and near-IR (NIR) fingerprint region is of special interest. The methodologies based on the photothermal techniques, mainly photoacoustic spectroscopy, have suitable characteristics for trace gas detection. Actually several laser-based methods have been reported because they are very sensitive [12–14]. For instance, photoacoustic spectroscopy is widely used in the detection of several gases in the ppbv and sub-ppbv concentration range [15–19].

A homemade CO₂ laser photoacoustic spectrometer has been developed to monitor gas emissions of several sources [20]. A continuous wave CO₂ infrared laser tunable over 80 different lines, between 9.2 and 10.6 μm, has been employed at the emission line of 10P(16) as excitation source for sulfur hexafluoride gas detection [21–23].

With the recent development of quantum-cascade lasers (QCLs), compact solid-state radiation sources are available, covering the important infrared (IR) region with specific molecular absorption lines. In addition, spectral regions known as atmospheric windows can be selected in which water vapor has a very low absorption coefficient. Another important advantage of QCLs in practical applications is that they work at near room temperature, whereas diode lasers such as lead salt lasers, which emit in the fundamental IR region, have to be cryogenically cooled.

Recent applications of QCLs clearly indicate their potential as tunable light sources in the mid-infrared, especially between 3 and 13 μm, with strong fundamental absorption bands. Current interest is based on the lack of other convenient coherent laser sources. In fact, it can be expected that QCLs will open new possibilities for real-time diagnostics of various molecular species in the 3–5 μm and 8–13 μm atmospheric windows [24].

Pulsed quantum-cascade distributed-feedback (QC-DFB) lasers provide quasi room temperature operation, combined with high spectral selectivity and sensitivity, real-time measurement capabilities, robustness, and compactness. For this reason, QCLs are ideal for the development of compact trace gas analyzers that are also suitable for field measurements. In recent years the detection of a series of important trace gases has been demonstrated with these devices [24–29].

In this work, we report on PA measurements of sulfur hexafluoride with two homemade Laser Photoacoustic Spectrometers, one equipped with a QC laser and another one with a CO₂ laser, aiming to make comparisons of performance between traditional CO₂ lasers (gas lasers) and new QC lasers (semiconductor lasers) for the detection of sulphur hexafluoride. In order to accomplish it, detection limit measurements were carried out. A value of 20 ppbv was achieved for the CO₂ laser system and a value of 50 ppbv was achieved for the QC laser system. The motivation of our research comes from the necessity for simple, sensitive, and spectrally selective devices capable of measuring this important greenhouse gas at trace levels.

The continuous wave infrared CO₂ laser, delivering a power level of 1.5 W, was employed as an excitation source at the emission line of 10P (16), λ = 10.5 μm. The initial concentration of 5 ppmv of SF₆ was diluted with pure nitrogen (zero gas) down to the lowest concentrations detected by the system. The acoustic and electronic noises were determined by blocking the laser light while keeping all other devices running. The value of the noise signal was typically 30 μV. As expected, a linear dependence of the photoacoustic signal on the sulfur hexafluoride concentration was observed. The measured PA signals and the fitted straight line are shown in (Figure 4). The smallest measured concentration was 20 ppbV for sulfur hexafluoride. As this function has a linear behavior we can extend this linearity to ppmv levels.

The pulsed quantum cascade laser (QCL), delivering a power of 1.12 mW at the wavelength of 10.5 μm, was used to excite the sulphur hexafluoride molecules. To determine the detection limit of the system, a dilution of a standard mixture of 5 ppmv of SF₆ in N₂ was carried out. Small concentrations of the gas were synthesized by using two electronic mass-flow controllers, one for N₂ (with full scale control of 200 sccm) and another for the investigated gas (SF₆) (with full scale control of 50 sccm). The electronic mass-flow controllers were connected in parallel to the gas inlet of the photoacoustic cell, and the initial SF₆ concentration of 5 ppmv was diluted with pure nitrogen (zero gas) down to the lowest concentrations detected by the system. The acoustic and the electronic noise were determined by blocking the laser light while keeping all other devices running. The value of the noise signal was typically 0.3 μV. As expected, a linear dependence of the photoacoustic signal on the sulfur hexafluoride concentration was found. The measured PA signals and the fitted straight line are shown in (Figure 5). The smallest measured concentration of sulfur hexafluoride was 50 ppbv. As this function has a linear behavior, we can extend this linearity to ppmv levels.

While the QCL provided a detection limit of 50 ppbv, the CO₂ laser reached a limit of 20 ppbv, therefore proving to be more sensitive. However, pulsed quantum-cascade distributed-feedback (QC-DFB) lasers provide quasi room temperature operation, combined with a high spectral selectivity and sensitivity, real-time measurement capabilities, robustness, and compactness, whereas CO₂ lasers are large, unstable and expensive in comparison. For this reason, QCLs are ideal for the development of compact trace gas analyzers that are also suitable for field measurements. We present an experimental setup where the QCLs is coupled to a differential photoacoustic cell that produces a photoacoustic signal with noise 100 times smaller (0.3 μV) when compared with the photoacoustic cell used in the experiment with CO₂ laser (30 μV). With the development of QCLs with continuous emission, rather than pulsed, greater power is provided, and along with proper alignments to optimize the laser beam it will be possible to achieve detection limits comparable to those obtained with the CO₂ laser.

For practical applications (industrial environments) that involve measurements in the atmosphere, the method proposed in this work must consider the presence of some chemical species that can alter the concentrations obtained for the SF₆ gas, such as CO₂ and water vapor. Thus one must use KOH and CaCl₂ filters so as to eliminate the possible interference from those species [34]. The typical concentration of SF₆ observed in the atmosphere is about 7 pptv, determined by NOAA (National Oceanic & Atmospheric Administration), on Mauna Loa, Hawaii, U.S.A. [35]. This value can be increased up to hundreds of ppbv to ppmv in certain processes and leaks in industrial environments.

The Photoacoustic Spectroscopy using CO₂ and quantum cascade lasers has proved to be extremely sensitive and selective for the detection of sulfur hexafluoride. This methodology allows measurements in anthropogenic sources that can reach concentrations higher than 20 ppbv, when using the CO₂ laser, and higher than 50 ppbv when using the pulsed quantum cascade distributed-feedback (QC-DFB) laser. These results are very promising and encourage further research.